1. Field of the Invention
The present invention relates to a light sensitive switching circuit and, more particularly, to a switching circuit which turns off upon detection of light.
2. Description of the Prior Art
The switching circuit of the above described type is used, for example, in combination with an outdoor lantern which turns on automatically when it becomes dark and turns off automatically when it becomes light.
The switching circuit generally includes a photosensor, such as phototransistor or CdS, for detecting the change in the brightness and a switching element for making and breaking the power supply to the load, such as a lantern. According to the prior art, the control of the switching element is done by a controller including a constant voltage source and various elements, such as transistors and resistors.
FIG. 1 shows a circuit diagram of a prior art light sensitive switching circuit which has hysteresis characteristics such that the lantern turns on when it becomes as dark as L1 and turns off when it becomes as light as L2, provided that L1 is darker than L2, as diagrammatically shown in FIG. 5. The operation of the prior art light sensitive switching circuit is described below.
When it is very dark, such as in the night time, the CdS shows a high resistance. Therefore, the voltage level at the junction A is low, resulting in a low voltage level at the junction B. Thus, transistor Tr121 turns on to permit a current flow through resistor R125. Thus, a voltage level at the junction C and also that at the junction D are high, thereby maintaining transistor Tr122 off. At this time, since the voltage level at the junction E is about 0 volts, transistor Tr123 is in the off condition. Accordingly, a current flows through resistor R130 to the gate of thyristor Th121, thereby turning thyristor Th121 on. Thus, through diode bridge DB121, a current flows to the base of triac T121, thereby turning triac T121 on. Accordingly, lantern L turns on.
When it is bright, such as in the day time, the CdS shows a low resistance. Therefore, the voltage level at the junction A and that at the junction B are high. Accordingly, transistor Tr21 is turned off. At this time, the junctions C and D have voltage levels divided by resistors R126, R127 and R125. Thus, by the voltage at the junction D, transistor Tr122 turns on. When transistor Tr122 is on, current flows through resistor R129, thereby increasing the voltage level at the junction E, and, in turn, turning transistor Tr123 on. Since transistor Tr123 receives to its collector a current from resistor R130 and also from the gate of thyristor Th121, thyristor Th121 turns off. When thyristor Th121 turns off, triac T121 also turns off, thereby cutting the power supply to lantern L.
According to the prior art switching circuit of FIG. 1, a constant voltage source defined by zener diode ZD, diode D, capacitor C122 and resistor R121 is necessary. Thus, because the constant voltage source is necessary, the circuit of FIG. 1 cannot employ such a circuit connection whereby lantern L and power source E are connected in series between power supply lines. Also, the control circuit for controlling triac T121 requires a number of circuit elements, such as transistors Tr121, Tr122 and Tr123, thyristor T121, etc. Thus, the circuit of the prior art requires many construction parts, and thus, results in a high manufacturing cost.